Using border gateway protocol to expose maximum segment identifier depth to an external application

ABSTRACT

A method implemented by a network device acting as a border gateway protocol (BGP) speaker, of exposing a maximum segment identifier depth (MSD) value of the network device is described. The method comprises encoding the MSD value into a BGP Link State (BGP-LS) extension message. The BGP-LS extension message includes a type, a length and a MSD value. The type indicates the type of the MSD value, the length indicates the length of the MSD value and the MSD value indicates a lowest MSD value supported by the network device for enabling segment routing. The method continues with transmitting the BGP-LS extension message including the type, the length, and the MSD value to a network controller, where the network controller is to use the MSD value to compute a segment routing path including the network device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/846,342, filed Sep. 4, 2015, which claims the benefit of U.S.Provisional Application No. 62/188,402, filed on Jul. 2, 2015, which arehereby incorporated by reference.

TECHNICAL FIELD

Embodiments of the invention relate to the field of packet networks, andmore specifically, to segment routing.

BACKGROUND ART

Segment Routing (SR) is a packet forwarding technology based on sourcerouting. A variety of use-cases for SR have been described in theInternet Engineering Task Force (IETF) draft“draft.filsfils-rtgwg-segment-routing-use-cases,” which is herebyincorporated by reference. An abstract routing model for SR is describedin an IETF draft “draft.filsfils-rtgwg-segment-routing,” which is herebyincorporated by reference. The IETF draft“draft.filsfils-rtgwg-segment-routing” describes the instantiation of SRusing Internet Protocol version 6 (IPv6) or Multiprotocol LabelSwitching (MPLS).

SUMMARY

A method implemented by a network device acting as a border gatewayprotocol (BGP) speaker, of exposing a maximum segment identifier depth(MSD) value of the network device is described. The method comprisesencoding the MSD value into a BGP Link State (BGP-LS) extension message.The BGP-LS extension message includes a type, a length and a MSD value.The type indicates the type of the MSD value, the length indicates thelength of the MSD value and the MSD value indicates a lowest MSD valuesupported by the network device for enabling segment routing. The methodcontinues with transmitting the BGP-LS extension message including thetype, the length, and the MSD value to a network controller, where thenetwork controller is to use the MSD value to compute a segment routingpath including the network device.

A network device acting as a border gateway protocol (BGP) speaker isdisclosed. The network device is to be coupled to a network controller.The network device comprises a processor and a memory, said memorycontaining instructions executable by the processor. The network deviceis operative to encode a maximum segment identifier depth (MSD) value ofthe network device into a BGP Link State (BGP-LS) extension message. TheBGP-LS extension message includes a type, a length and a MSD value. Thetype indicates the type of the MSD value, the length indicates thelength of the MSD value and the MSD value indicates a lowest MSD valuesupported by the network device for enabling segment routing. Thenetwork device is further operative to transmit the BGP-LS extensionmessage including the type, the length, and the MSD value to the networkcontroller, wherein the network controller is to use the MSD value tocompute a segment routing path including the network device.

A non-transitory machine-readable storage medium is disclosed. Thenon-transitory machine-readable storage medium provides instructionsthat, if executed by a processor of a network device acting as a bordergateway protocol (BGP) speaker and coupled with a network controller,will cause said processor to perform operations. The operations compriseencoding a maximum segment identifier depth (MSD) value of the networkdevice into a BGP Link State (BGP-LS) extension message. The BGP-LSextension message includes a type, a length and a MSD value. The typeindicates the type of the MSD value, the length indicates the length ofthe MSD value and the MSD value indicates a lowest MSD value supportedby the network device for enabling segment routing. The operationsfurther comprise transmitting the BGP-LS extension message including thetype, the length, and the MSD value to the network controller, whereinthe network controller is to use the MSD value to compute a segmentrouting path including the network device.

A method in a network controller is disclosed. The method comprisesreceiving from a network device acting as a border gateway protocol(BGP) speaker, a BGP Link State (BGP-LS) extension message. The methodcontinues with decoding the BGP-LS extension message, to extract amaximum segment identifier depth (MSD) value of the network device. TheBGP-LS extension message includes a type which indicates the type of theMSD value, a length which indicates the length of the MSD value and theMSD value which indicates a lowest MSD value supported by the networkdevice for enabling segment routing. The method further includescomputing, using the MSD value, a segment routing (SR) path includingthe network device, where the SR path has a label stack depth that islower than or equal to the MSD value.

A network controller is disclosed. The network controller comprises aprocessor and a memory, said memory containing instructions executableby the processor. The network controller is operative to receive from anetwork device acting as a border gateway protocol (BGP) speaker, a BGPLink State (BGP-LS) extension message. The network controller is furtheroperative to the BGP-LS extension message, to extract a maximum segmentidentifier depth (MSD) value of the network device. The BGP-LS extensionmessage includes a type which indicates the type of the MSD value, alength which indicates the length of the MSD value and the MSD valuewhich indicates a lowest MSD value supported by the network device forenabling segment routing. The network controller is further operative tocompute, using the MSD value, a segment routing (SR) path including thenetwork device, wherein the SR path has a label stack depth that islower than or equal to the MSD value.

A non-transitory machine-readable storage medium is disclosed. Thenon-transitory machine-readable storage medium provides instructionsthat, if executed by a processor of a network controller, will causesaid processor to perform operations. The operations comprise receivingfrom a network device acting as a border gateway protocol (BGP) speaker,a BGP Link State (BGP-LS) extension message. The operations furthercomprise decoding the BGP-LS extension message, to extract a maximumsegment identifier depth (MSD) value of the network device. The BGP-LSextension message includes a type which indicates the type of the MSDvalue, a length which indicates the length of the MSD value and the MSDvalue which indicates a lowest MSD value supported by the network devicefor enabling segment routing. The operations further comprise computing,using the MSD value, a segment routing (SR) path including the networkdevice, wherein the SR path has a label stack depth that is lower thanor equal to the MSD value.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 is a block diagram illustrating SR network enabling transmissionof maximum segment identifier (SID) depth (MSD) to external applicationsaccording to some embodiments of the invention.

FIG. 2 illustrates a flow diagram of operations performed in a networkdevice for exposing an MSD value in accordance with some embodiments ofthe invention.

FIG. 3 illustrates a flow diagram of operations performed in a networkcontroller in accordance with some embodiments of the invention.

FIG. 4A illustrates an exemplary opaque node attribute Type-Length-Value(TLV) including a node MSD value of a network device in accordance withsome embodiments.

FIG. 4B illustrates an exemplary opaque link attribute TLV including alink MSD value of a network device in accordance with some embodiments.

FIG. 5A illustrates connectivity between network devices (NDs) within anexemplary network, as well as three exemplary implementations of theNDs, according to some embodiments of the invention.

FIG. 5B illustrates an exemplary way to implement a special-purposenetwork device according to some embodiments of the invention.

FIG. 5C illustrates various exemplary ways in which virtual networkelements (VNEs) may be coupled according to some embodiments of theinvention.

FIG. 5D illustrates a network with a single network element (NE) on eachof the NDs, and within this straight forward approach contrasts atraditional distributed approach (commonly used by traditional routers)with a centralized approach for maintaining reachability and forwardinginformation (also called network control), according to some embodimentsof the invention.

FIG. 5E illustrates the simple case of where each of the NDs implementsa single NE, but a centralized control plane has abstracted multiple ofthe NEs in different NDs into (to represent) a single NE in one of thevirtual network(s), according to some embodiments of the invention.

FIG. 5F illustrates a case where multiple VNEs are implemented ondifferent NDs and are coupled to each other, and where a centralizedcontrol plane has abstracted these multiple VNEs such that they appearas a single VNE within one of the virtual networks, according to someembodiments of the invention.

FIG. 6 illustrates a general purpose control plane device withcentralized control plane (CCP) software 650), according to someembodiments of the invention.

DETAILED DESCRIPTION

The following description describes methods and apparatus for exposing amaximum segment identifier depth (MSD) value associated with a networkdevice. In the following description, numerous specific details such aslogic implementations, opcodes, means to specify operands, resourcepartitioning/sharing/duplication implementations, types andinterrelationships of system components, and logicpartitioning/integration choices are set forth in order to provide amore thorough understanding of the present invention. It will beappreciated, however, by one skilled in the art that the invention maybe practiced without such specific details. In other instances, controlstructures, gate level circuits and full software instruction sequenceshave not been shown in detail in order not to obscure the invention.Those of ordinary skill in the art, with the included descriptions, willbe able to implement appropriate functionality without undueexperimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

Bracketed text and blocks with dashed borders (e.g., large dashes, smalldashes, dot-dash, and dots) may be used herein to illustrate optionaloperations that add additional features to embodiments of the invention.However, such notation should not be taken to mean that these are theonly options or optional operations, and/or that blocks with solidborders are not optional in certain embodiments of the invention.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.“Coupled” is used to indicate that two or more elements, which may ormay not be in direct physical or electrical contact with each other,co-operate or interact with each other. “Connected” is used to indicatethe establishment of communication between two or more elements that arecoupled with each other.

An electronic device or a computing device (e.g., an end station, anetwork device) stores and transmits (internally and/or with otherelectronic devices over a network) code (composed of softwareinstructions) and data using machine-readable media, such asnon-transitory machine-readable media (e.g., machine-readable storagemedia such as magnetic disks; optical disks; read only memory; flashmemory devices; phase change memory) and transitory machine-readabletransmission media (e.g., electrical, optical, acoustical or other formof propagated signals—such as carrier waves, infrared signals). Inaddition, such electronic devices include hardware, such as a set of oneor more processors coupled to one or more other components—e.g., one ormore non-transitory machine-readable storage media (to store code and/ordata) and network connections (to transmit code and/or data usingpropagating signals), as well as user input/output devices (e.g., akeyboard, a touchscreen, and/or a display) in some cases. The couplingof the set of processors and other components is typically through oneor more interconnects within the electronic devices (e.g., busses andpossibly bridges). Thus, a non-transitory machine-readable medium of agiven electronic device typically stores instructions for execution onone or more processors of that electronic device. One or more parts ofan embodiment of the invention may be implemented using differentcombinations of software, firmware, and/or hardware.

As used herein, a network device (e.g., a router, switch, bridge) is apiece of networking equipment, including hardware and software, whichcommunicatively interconnects other equipment on the network (e.g.,other network devices, end stations). Some network devices are “multipleservices network devices” that provide support for multiple networkingfunctions (e.g., routing, bridging, switching, Layer 2 aggregation,session border control, Quality of Service, and/or subscribermanagement), and/or provide support for multiple application services(e.g., data, voice, and video). Subscriber end stations (e.g., servers,workstations, laptops, netbooks, palm tops, mobile phones, smartphones,multimedia phones, Voice Over Internet Protocol (VOIP) phones, userequipment, terminals, portable media players, GPS units, gaming systems,set-top boxes) access content/services provided over the Internet and/orcontent/services provided on virtual private networks (VPNs) overlaid on(e.g., tunneled through) the Internet. The content and/or services aretypically provided by one or more end stations (e.g., server endstations) belonging to a service or content provider or end stationsparticipating in a peer-to-peer (P2P) service, and may include, forexample, public webpages (e.g., free content, store fronts, searchservices), private webpages (e.g., username/password accessed webpagesproviding email services), and/or corporate networks over VPNs.Typically, subscriber end stations are coupled (e.g., through customerpremise equipment coupled to an access network (wired or wirelessly)) toedge network devices, which are coupled (e.g., through one or more corenetwork devices) to other edge network devices, which are coupled toother end stations (e.g., server end stations).

Network devices are commonly separated into a control plane and a dataplane (sometimes referred to as a forwarding plane or a media plane). Inthe case that the network device is a router (or is implementing routingfunctionality), the control plane typically determines how data (e.g.,packets) is to be routed (e.g., the next hop for the data and theoutgoing port for that data), and the data plane is in charge offorwarding that data. For example, the control plane typically includesone or more routing protocols (e.g., an exterior gateway protocol suchas Border Gateway Protocol (BGP), Interior Gateway Protocol(s) (IGP)(e.g., Open Shortest Path First (OSPF), Intermediate System toIntermediate System (IS-IS), Routing Information Protocol (RIP)), LabelDistribution Protocol (LDP), Resource Reservation Protocol (RSVP)) thatcommunicate with other network devices to exchange routes and selectthose routes based on one or more routing metrics.

A control plane or a component external to the network may performcomputations based on the network topology and current state of theconnections within the network, including traffic engineeringinformation. For example in order to determine segment routing paths,the processing component (e.g., control plane, SDN controller, PathComputation Element (PCE), etc.) needs to gather information about thetopologies and capabilities of each network device of the SR network inorder to properly configure it. In Segment Routing (SR), a network node(e.g., a SR-capable network device) steers a packet through the networkby utilizing a set of instructions, herein referred to as segments. Thesegments are included as part of an SR header which has been prependedonto the packet at the ingress of an SR network. A segment can representany topological or service instruction. SR architecture can be appliedto the MPLS data plane with no change in the forwarding plane. SR, whenapplied to the MPLS data plane, provides the ability to tunnel services(e.g., Virtual Private Network (VPN), Virtual Private Local LAN Service(VPLS), Virtual Private Wire Service (VPWS)) from an ingress LabelSwitched Router (LSR) to an egress LSR, without any protocol other thanIS-IS or OSPF. A segment is encoded as an MPLS label. An ordered list ofsegments is encoded as a stack of labels. The next segment to process ison the top of the stack. Upon completion of a segment, the correspondinglabel is popped from the stack.

In particular, a control plane needs to gather information about themaximum segment identifier (SID) depth (MSD) of each network node beingconfigured, such that the controller does not configure the network nodewith a path/tunnel which would have a SID (or label stack) deeper thanthe network node is capable of imposing. The “Maximum SID Depth” (MSD)specifies the maximum number of SIDs (i.e., segments or label stacks)that a SR network node is capable of imposing on a packet.

In one approach, the control plane may use the Path Computation ElementCommunication Protocol (PCEP) to retrieve the MSD of a SR network node.PCEP is a set of rules that allows a Path Computation Client (PCC)(e.g., the network node being configured by the control plane) torequest path computations from Path Computation Elements (PCEs) (e.g.,the control plane). According to PCEP, the MSD is transferred to thecontrol plane via a PCEP extension “SR PCE Capability TLV/METRICObject,” as discussed in “draft-ietf-pce-segment-routing” (which ishereby incorporated by reference).

In another approach, the MSD value for each network node may betransferred to the control plane with the Interior Gateway Protocol(s)(IGP). In this approach, the sub-TLV, called RLSDC sub-TLV, is definedto advertise the capability of the network node to read the maximumlabel stack depth (i.e., MSD of the network device) as defined for OSPFand IS-IS in “draft-ietf-ospf-mpls-elc” and “draft-ietf-isis-mpls-elc”respectively (which are hereby incorporated by reference).

Challenges with Exposing MSD to a Network Controller

Although PCEP may be used to expose the MSD of a network node to anothernetwork device (e.g., the control plane), when PCEP is not supported bya network node (in particular the head-end of the SR path/tunnel), or ifthe network node does not participate in IGP routing, the control planedoes not have any method to learn the MSD of the network node beingconfigured. This results in the control plane making arbitrary decisionswith regards to the label stack depth of the network node beingconfigured, which may result in an erroneous configuration of thenetwork node, leading to packet drops. In addition, PCEP and IGP do notallow a control plane to obtain the MSD of a link within the networknode, and only allows the control plane to get the global node MSD.

The embodiments presented herein overcome the limitations of theprevious approaches. According to some embodiments, a method implementedby a network device acting as a border gateway protocol (BGP) speaker,of transmitting a maximum segment identifier depth (MSD) valueassociated with the network device, to a network controller isdisclosed. In these embodiments, a MSD value of the network device isencoded into a BGP Link State (BGP-LS) extension message. The BGP-LSextension message includes, a type that indicates the type of the MSDvalue, a length which indicate the length of the MSD value and the MSDvalue that indicates a lowest node MSD value of the network device. Inother embodiments, the MSD value is a link MSD value indicative of alowest MSD value for each link of the network device. The BGP-LSextension message is then transmitted to the network controller, whereinthe network controller is to use the MSD value when calculating thesegment routing path including the network device.

BGP-LS as described in “draft-ietf-idr-ls-distribution-11” (which ishereby incorporated by reference), describes a mechanism by which linksstate and traffic engineering information can be collected from networksand shared with external components (e.g., control plane, centralizedcontroller, PCE, etc.) using the BGP routing protocol.

FIG. 1 is a block diagram illustrating SR network enabling transmissionof maximum segment identifier (SID) depth (MSD) to external applicationsaccording to some embodiments.

FIG. 1, illustrates a centralized control plane 176 coupled with a dataplane 180 (sometimes referred to as the infrastructure layer, networkforwarding plane, or forwarding plane (which should not be confused witha ND forwarding plane)) that includes the network elements (NEs) 170A-H(sometimes referred to as routers, switches, forwarding elements, dataplane elements, or nodes). The centralized control plane 176 includes anetwork controller 178, which includes a centralized reachability andforwarding information module (not illustrated) that determines thereachability within the network and distributes the forwardinginformation to the NEs 170A-H of the data plane 180. In theseembodiments, the network intelligence is centralized in the centralizedcontrol plane 176 executing on electronic devices that are typicallyseparate from the NDs.

FIG. 1 also shows that the centralized control plane 176 has a northbound interface 184 to an application layer 186, in which residesapplication(s) 188. The centralized control plane 176 has the ability toform virtual networks (sometimes referred to as a logical forwardingplane, network services, or overlay networks (with the NEs 170A-H of thedata plane 180 being the underlay network)) for the application(s) 188.Thus, the centralized control plane 176 maintains a global view of allNDs and configured NEs/VNEs, and it maps the virtual networks to theunderlying NDs efficiently (including maintaining these mappings as thephysical network changes either through hardware (ND, link, or NDcomponent) failure, addition, or removal). Each network element NE170A-H may be implemented by one or more network devices, as it isdescribed in further detail below.

The NE 170E includes a MSD value which is an indication of thecapabilities of the node and/or a link of the node. In some embodiments,the MSD value is a numeric value in the range of [0, 254] that isstatically configured in the NE 170E. In alternative embodiments, theMSD value is redistributed from an IGP node. In some embodiments, theMSD value is uniquely associated with the network device.

The MSD value may be a node MSD value, which indicates the maximumnumber of labels (SIDs) supported by the NE 170E. In some embodiments, avalue of 0 means that NE 170E is not operable to push an SR stack of anylength and should not be used for such functionality. Alternatively, theMSD value may be a link MSD value which indicates a maximum number oflabels (SIDs) supported by a link of the NE 170E. In some embodiments, avalue of 0 means the link is not operable to push an SR stack of anylength and should not be used for such functionality.

The NE 170E and the network controller 178 are configured to include BGPand to act as BGP speakers in the network 100. During a peeringhandshake, OPEN messages are exchanged between the two BGP speakers (NE170E and network controller 178). NE 170E and the network controller 178negotiate capabilities of the session. In order for the two BGP speakers(NE 170E and network controller 178) to exchange Link-State NetworkLayer Reachability Information (NLRI), they use BGP CapabilitiesAdvertisement to ensure that they both are capable of properlyprocessing such NLRI. The negotiation of the capabilities includes theverification that each one of the NE 170E and the network controller 178is capable of supporting the new functionality of BGP-LS presentedherein for transmission of a BGP-LS extension message in which the MSDvalue of the NE 170E is encoded.

Following the negotiation of the capabilities, NE 170E, at task box (1),encodes the MSD value into a BGP Link State (BGP-LS) extension messageto be transmitted to the network controller 178. The BGP-LS extensionmessage includes a type which indicates the type of the MSD value, alength which indicates the length of the MSD value and an MSD valuewhich indicates a lowest MSD value supported by the network node forenabling a segment routing path.

In some embodiments, the MSD value is the node MSD value of NE 170E. Insome of these embodiments, the node MSD value is encoded in an OpaqueNode Attribute Type-Length-Value (TLV) of BGP-LS. The Opaque NodeAttribute TLV is an envelope that carries node attribute TLVs advertisedby a network element. In embodiments of the invention, the Opaque NodeAttribute TLV is used to carry the node MSD value of NE 170E. The NE170E uses the Opaque Node Attribute TLV to advertise to other BGPspeakers (e.g., network controller 178) its associated node MSD value.FIG. 4A illustrates an exemplary opaque node attribute TLV including anode MSD value of a network element in accordance with some embodiments.The opaque node attribute TLV is a container 400 of type TLV(Type-Length-Value). The type 410 includes a value identifying the typeof the value transmitted within the container 400, the length 412specifies the length of the value transmitted within the container 400,and the opaque node attribute 414 includes the MSD value associated withthe NE 170E. In some embodiments, the MSD value is a value from therange [0, 254] and indicates the maximum number of labels (SIDs)supported by the NE 170E.

In other embodiments, the MSD value is a link MSD of NE 170E. The OpaqueLink Attribute TLV is an envelope that carries link attribute TLVsadvertised by a network element. The NE 170E uses the Opaque LinkAttribute TLV to advertise to other BGP speakers (e.g., networkcontroller 178) its associated link MSD. FIG. 4B illustrates anexemplary opaque link attribute TLV including a link MSD value of anetwork element in accordance with some embodiments. The opaque nodeattribute TLV is a container 402 of type TLV (Type-Length-Value). Thetype 416 includes a value identifying the type of the value transmittedwithin the container 402 (e.g., link MSD type), the length 418 specifiesthe length of the value transmitted within the container 402, and theopaque link attribute 420 includes the link MSD value associated withthe NE 170E. In some embodiments, the link MSD value is a value from therange [0, 254] and indicates the lowest maximum number of labels (SIDs)supported by a link of the NE 170E.

Referring back to FIG. 1, at task box (2), the MSD value of NE 170E istransmitted to the network controller 178. In some embodiments,following the encoding of the MSD value in a BGP-LS extension message(e.g., opaque node attribute TLV or opaque link attribute TLV), themessage is advertised to all BGP speakers coupled with NE 170E.

At task box (3), the network controller 178 decodes the BGP-LS extensionmessage, to extract a maximum segment identifier depth (MSD) value ofthe network device. The BGP-LS extension message includes a type whichindicates the type of the MSD value, a length which indicates the lengthof the MSD value and an MSD value which indicates a lowest MSD valuesupported by the network device for enabling segment routing. In someembodiments, once the message is decoded and the MSD value is extracted,the network controller 178 uses the MSD value to compute a segmentrouting path which includes NE 170E such that the label stack depth ofthe routing path computed does not exceed the MSD value supported by thenetwork element 170E. In some embodiments, the MSD value is uniquelyassociated with the network device.

In some embodiments, the network element 170E is a network node which isthe head-end of a SR tunnel/path. Thus the MSD value sent to the networkcontroller 178 enables the network controller to compute a SR pathallowing the network element 170E to push a complete label (SID) stackof maximum depth equal to the MSD value.

Thus the embodiments presented herein describe an efficient way ofexposing the MSD value of a network node to external applications (e.g.,PCE/SDN controller, network controller) with the use of BGP-LSextensions. The embodiments enable an external application to receiveMSD information related to a network element at a finer granularity thanprior approaches. Contrary to the prior approaches which enable anetwork device to only expose the node MSD value of the network node,the mechanisms described with reference to the FIGS. 1-4B enable anetwork device to expose its associated MSD values at the link level inaddition to being able to expose the node MSD value.

The operations in the flow diagrams will be described with reference tothe exemplary embodiments of the other figures. However, it should beunderstood that the operations of the flow diagrams can be performed byembodiments of the invention other than those discussed with referenceto the other figures, and the embodiments of the invention discussedwith reference to these other figures can perform operations differentthan those discussed with reference to the flow diagrams.

FIG. 2 illustrates a flow diagram of operations 200 performed in anetwork device for exposing an MSD value of the network device inaccordance with some embodiments. At block 202, the NE 170E encodes theMSD value into a BGP Link State (BGP-LS) extension message. The BGP-LSextension message includes a type, a length and a MSD value, and wherethe type indicates the type of the MSD value, the length indicates thelength of the MSD value and the MSD value indicates a lowest MSD valuesupported by the network device for enabling segment routing. Flow thenmoves to block 203.

At block 203, NE 170E transmits the BGP-LS extension message includingthe MSD value to the network controller 178. The network controller 178is operative to use the received MSD value to compute a segment routingpath including the network device. In some embodiments, the networkelement 170E is a network node which is the head-end of a SRtunnel/path. Thus the MSD value sent to the controller enables thecontroller to compute a SR path allowing the network element to push acomplete label (SID) stack of maximum depth equal to the MSD value.

FIG. 3 illustrates a flow diagram of operations performed in a networkcontroller in accordance with some embodiments. At block 302, thenetwork controller 178 receives from a network device (e.g., NE 170E)acting as a border gateway protocol (BGP) speaker, a BGP Link State (LS)extension message. Flow then moves to block 304.

At block 304, the network controller 178 decodes the BGP-LS extensionmessage, to extract a maximum segment identifier depth (MSD) value ofthe network device. The BGP-LS extension message includes a type whichindicates the type of the MSD value, a length which indicates the lengthof the MSD value and a MSD value which indicates a lowest MSD valuesupported by the network device for enabling segment routing. In someembodiments, the MSD value is uniquely associated with the networkdevice.

At block 306, the network controller computes, using the extracted MSDvalue, a segment routing (SR) path including the network device, wherethe SR path has a label stack depth that is lower than or equal to theMSD value. In some embodiments, the network element 170E is a networknode which is the head-end of a SR tunnel/path. Thus the MSD value sentto the controller enables the controller to compute a SR path allowingthe network node to push a complete label (SID) stack of maximum depthequal to the MSD value.

While embodiments of the invention have been described with a networkdevice transmitting an MSD value encoded in a BGP-LS extension messageto a network controller of a control plane, the invention is not solimited. Alternative embodiments could be implemented such that the MSDvalue is transmitted to any type of network element coupled with thenetwork device and operative to decode a BGP-LS extension messageincluding the MSD value.

Architecture

Typically, a network device includes a set of one or more line cards, aset of one or more control cards, and optionally a set of one or moreservice cards (sometimes referred to as resource cards). These cards arecoupled together through one or more interconnect mechanisms (e.g., afirst full mesh coupling the line cards and a second full mesh couplingall of the cards). The set of line cards make up the data plane, whilethe set of control cards provide the control plane and exchange packetswith external network devices through the line cards. The set of servicecards can provide specialized processing (e.g., Layer 4 to Layer 7services (e.g., firewall, Internet Protocol Security (IPsec), IntrusionDetection System (IDS), peer-to-peer (P2P), Voice over IP (VoIP) SessionBorder Controller, Mobile Wireless Gateways (Gateway General PacketRadio Service (GPRS) Support Node (GGSN), Evolved Packet Core (EPC)Gateway)). By way of example, a service card may be used to terminateIPsec tunnels and execute the attendant authentication and encryptionalgorithms.

FIG. 5A illustrates connectivity between network devices (NDs) within anexemplary network, as well as three exemplary implementations of theNDs, according to some embodiments of the invention. FIG. 5A shows NDs500A-H, and their connectivity by way of lines between A-B, B-C, C-D,D-E, E-F, F-G, and A-G, as well as between H and each of A, C, D, and G.These NDs are physical devices, and the connectivity between these NDscan be wireless or wired (often referred to as a link). An additionalline extending from NDs 500A, E, and F illustrates that these NDs act asingress and egress points for the network (and thus, these NDs aresometimes referred to as edge NDs; while the other NDs may be calledcore NDs).

Two of the exemplary ND implementations in FIG. 5A are: 1) aspecial-purpose network device 502 that uses custom application—specificintegrated—circuits (ASICs) and a proprietary operating system (OS); and2) a general purpose network device 504 that uses common off-the-shelf(COTS) processors and a standard OS.

The special-purpose network device 502 includes networking hardware 510comprising compute resource(s) 512 (which typically include a set of oneor more processors), forwarding resource(s) 514 (which typically includeone or more ASICs and/or network processors), and physical networkinterfaces (NIs) 516 (sometimes called physical ports), as well asnon-transitory machine readable storage media 518 having stored thereinnetworking software 520. A physical NI is hardware in a ND through whicha network connection (e.g., wirelessly through a wireless networkinterface controller (WNIC) or through plugging in a cable to a physicalport connected to a network interface controller (NIC)) is made, such asthose shown by the connectivity between NDs 500A-H. During operation,the BGP-LS MSD value encoder 520 may be executed by the networkinghardware 510 to instantiate a set of one or more networking softwareinstance(s) 522 which include BGP-LS MSD value encoder instances 533A-R.Each of the networking software instance(s) 522, and that part of thenetworking hardware 510 that executes that network software instance (beit hardware dedicated to that networking software instance and/or timeslices of hardware temporally shared by that networking softwareinstance with others of the networking software instance(s) 522), form aseparate virtual network element 530A-R. During operation, the BGP-LSMSD value encoder is operative to perform operations described withreference to FIGS. 1, 2, and 4A-4B. Each of the virtual networkelement(s) (VNEs) 530A-R includes a control communication andconfiguration module 532A-R (sometimes referred to as a local controlmodule or control communication module) and forwarding table(s) 534A-R,such that a given virtual network element (e.g., 530A) includes thecontrol communication and configuration module (e.g., 532A), a set ofone or more forwarding table(s) (e.g., 534A), and that portion of thenetworking hardware 510 that executes the virtual network element (e.g.,530A).

The special-purpose network device 502 is often physically and/orlogically considered to include: 1) a ND control plane 524 (sometimesreferred to as a control plane) comprising the compute resource(s) 512that execute the control communication and configuration module(s)532A-R; and 2) a ND forwarding plane 526 (sometimes referred to as aforwarding plane, a data plane, or a media plane) comprising theforwarding resource(s) 514 that utilize the forwarding table(s) 534A-Rand the physical NIs 516. By way of example, where the ND is a router(or is implementing routing functionality), the ND control plane 524(the compute resource(s) 512 executing the control communication andconfiguration module(s) 532A-R) is typically responsible forparticipating in controlling how data (e.g., packets) is to be routed(e.g., the next hop for the data and the outgoing physical NI for thatdata) and storing that routing information in the forwarding table(s)534A-R, and the ND forwarding plane 526 is responsible for receivingthat data on the physical NIs 516 and forwarding that data out theappropriate ones of the physical NIs 516 based on the forwardingtable(s) 534A-R.

FIG. 5B illustrates an exemplary way to implement the special-purposenetwork device 502 according to some embodiments of the invention. FIG.5B shows a special-purpose network device including cards 538 (typicallyhot pluggable). While in some embodiments the cards 538 are of two types(one or more that operate as the ND forwarding plane 526 (sometimescalled line cards), and one or more that operate to implement the NDcontrol plane 524 (sometimes called control cards)), alternativeembodiments may combine functionality onto a single card and/or includeadditional card types (e.g., one additional type of card is called aservice card, resource card, or multi-application card). A service cardcan provide specialized processing (e.g., Layer 4 to Layer 7 services(e.g., firewall, Internet Protocol Security (IPsec) (RFC 4301 and 4309),Secure Sockets Layer (SSL)/Transport Layer Security (TLS), IntrusionDetection System (IDS), peer-to-peer (P2P), Voice over IP (VoIP) SessionBorder Controller, Mobile Wireless Gateways (Gateway General PacketRadio Service (GPRS) Support Node (GGSN), Evolved Packet Core (EPC)Gateway)). By way of example, a service card may be used to terminateIPsec tunnels and execute the attendant authentication and encryptionalgorithms. These cards are coupled together through one or moreinterconnect mechanisms illustrated as backplane 536 (e.g., a first fullmesh coupling the line cards and a second full mesh coupling all of thecards).

Returning to FIG. 5A, the general purpose network device 504 includeshardware 540 comprising a set of one or more processor(s) 542 (which areoften COTS processors) and network interface controller(s) 544 (NICs;also known as network interface cards) (which include physical NIs 546),as well as non-transitory machine readable storage media 548 havingstored therein software 550. During operation, the processor(s) 542execute the software 550 to instantiate one or more sets of one or moreapplications 564A-R. While one embodiment does not implementvirtualization, alternative embodiments may use different forms ofvirtualization—represented by a virtualization layer 554 and softwarecontainers 562A-R. For example, one such alternative embodimentimplements operating system-level virtualization, in which case thevirtualization layer 554 represents the kernel of an operating system(or a shim executing on a base operating system) that allows for thecreation of multiple software containers 562A-R that may each be used toexecute one of the sets of applications 564A-R. In this embodiment, themultiple software containers 562A-R (also called virtualization engines,virtual private servers, or jails) are each a user space instance(typically a virtual memory space); these user space instances areseparate from each other and separate from the kernel space in which theoperating system is run; the set of applications running in a given userspace, unless explicitly allowed, cannot access the memory of the otherprocesses. Another such alternative embodiment implements fullvirtualization, in which case: 1) the virtualization layer 554represents a hypervisor (sometimes referred to as a virtual machinemonitor (VMM)) or a hypervisor executing on top of a host operatingsystem; and 2) the software containers 562A-R each represent a tightlyisolated form of software container called a virtual machine that is runby the hypervisor and may include a guest operating system. A virtualmachine is a software implementation of a physical machine that runsprograms as if they were executing on a physical, non-virtualizedmachine; and applications generally do not know they are running on avirtual machine as opposed to running on a “bare metal” host electronicdevice, though some systems provide para-virtualization which allows anoperating system or application to be aware of the presence ofvirtualization for optimization purposes.

The instantiation of the one or more sets of one or more applications564A-R, as well as the virtualization layer 554 and software containers562A-R if implemented, are collectively referred to as softwareinstance(s) 552. Each set of applications 564A-R, corresponding softwarecontainer 562A-R if implemented, and that part of the hardware 540 thatexecutes them (be it hardware dedicated to that execution and/or timeslices of hardware temporally shared by software containers 562A-R),forms a separate virtual network element(s) 560A-R.

The virtual network element(s) 560A-R perform similar functionality tothe virtual network element(s) 530A-R—e.g., similar to the controlcommunication and configuration module(s) 532A and forwarding table(s)534A (this virtualization of the hardware 540 is sometimes referred toas network function virtualization (NFV)). Thus, NFV may be used toconsolidate many network equipment types onto industry standard highvolume server hardware, physical switches, and physical storage, whichcould be located in Data centers, NDs, and customer premise equipment(CPE). However, different embodiments of the invention may implement oneor more of the software container(s) 562A-R differently. For example,while embodiments of the invention are illustrated with each softwarecontainer 562A-R corresponding to one VNE 560A-R, alternativeembodiments may implement this correspondence at a finer levelgranularity (e.g., line card virtual machines virtualize line cards,control card virtual machine virtualize control cards, etc.); it shouldbe understood that the techniques described herein with reference to acorrespondence of software containers 562A-R to VNEs also apply toembodiments where such a finer level of granularity is used.

In certain embodiments, the virtualization layer 554 includes a virtualswitch that provides similar forwarding services as a physical Ethernetswitch. Specifically, this virtual switch forwards traffic betweensoftware containers 562A-R and the NIC(s) 544, as well as optionallybetween the software containers 562A-R; in addition, this virtual switchmay enforce network isolation between the VNEs 560A-R that by policy arenot permitted to communicate with each other (e.g., by honoring virtuallocal area networks (VLANs)).

The third exemplary ND implementation in FIG. 5A is a hybrid networkdevice 506, which includes both custom ASICs/proprietary OS and COTSprocessors/standard OS in a single ND or a single card within an ND. Incertain embodiments of such a hybrid network device, a platform VM(i.e., a VM that that implements the functionality of thespecial-purpose network device 502) could provide forpara-virtualization to the networking hardware present in the hybridnetwork device 506.

Regardless of the above exemplary implementations of an ND, when asingle one of multiple VNEs implemented by an ND is being considered(e.g., only one of the VNEs is part of a given virtual network) or whereonly a single VNE is currently being implemented by an ND, the shortenedterm network element (NE) is sometimes used to refer to that VNE. Alsoin all of the above exemplary implementations, each of the VNEs (e.g.,VNE(s) 530A-R, VNEs 560A-R, and those in the hybrid network device 506)receives data on the physical NIs (e.g., 516, 546) and forwards thatdata out the appropriate ones of the physical NIs (e.g., 516, 546). Forexample, a VNE implementing IP router functionality forwards IP packetson the basis of some of the IP header information in the IP packet;where IP header information includes source IP address, destination IPaddress, source port, destination port (where “source port” and“destination port” refer herein to protocol ports, as opposed tophysical ports of a ND), transport protocol (e.g., user datagramprotocol (UDP) (RFC 768, 2460, 2675, 4113, and 5405), TransmissionControl Protocol (TCP) (RFC 793 and 1180), and differentiated services(DSCP) values (RFC 2474, 2475, 2597, 2983, 3086, 3140, 3246, 3247, 3260,4594, 5865, 3289, 3290, and 3317).

FIG. 5C illustrates various exemplary ways in which VNEs may be coupledaccording to some embodiments of the invention. FIG. 5C shows VNEs570A.1-570A.P (and optionally VNEs 570A.Q-570A.R) implemented in ND 500Aand VNE 570H.1 in ND 500H. In FIG. 5C, VNEs 570A.1-P are separate fromeach other in the sense that they can receive packets from outside ND500A and forward packets outside of ND 500A; VNE 570A.1 is coupled withVNE 570H.1, and thus they communicate packets between their respectiveNDs; VNE 570A.2-570A.3 may optionally forward packets between themselveswithout forwarding them outside of the ND 500A; and VNE 570A.P mayoptionally be the first in a chain of VNEs that includes VNE 570A.Qfollowed by VNE 570A.R (this is sometimes referred to as dynamic servicechaining, where each of the VNEs in the series of VNEs provides adifferent service—e.g., one or more layer 4-7 network services). WhileFIG. 5C illustrates various exemplary relationships between the VNEs,alternative embodiments may support other relationships (e.g.,more/fewer VNEs, more/fewer dynamic service chains, multiple differentdynamic service chains with some common VNEs and some different VNEs).

The NDs of FIG. 5A, for example, may form part of the Internet or aprivate network; and other electronic devices (not shown; such as enduser devices including workstations, laptops, netbooks, tablets, palmtops, mobile phones, smartphones, phablets, multimedia phones, VoiceOver Internet Protocol (VOIP) phones, terminals, portable media players,GPS units, wearable devices, gaming systems, set-top boxes, Internetenabled household appliances) may be coupled to the network (directly orthrough other networks such as access networks) to communicate over thenetwork (e.g., the Internet or virtual private networks (VPNs) overlaidon (e.g., tunneled through) the Internet) with each other (directly orthrough servers) and/or access content and/or services. Such contentand/or services are typically provided by one or more servers (notshown) belonging to a service/content provider or one or more end userdevices (not shown) participating in a peer-to-peer (P2P) service, andmay include, for example, public webpages (e.g., free content, storefronts, search services), private webpages (e.g., username/passwordaccessed webpages providing email services), and/or corporate networksover VPNs. For instance, end user devices may be coupled (e.g., throughcustomer premise equipment coupled to an access network (wired orwirelessly)) to edge NDs, which are coupled (e.g., through one or morecore NDs) to other edge NDs, which are coupled to electronic devicesacting as servers. However, through compute and storage virtualization,one or more of the electronic devices operating as the NDs in FIG. 5Amay also host one or more such servers (e.g., in the case of the generalpurpose network device 504, one or more of the software containers562A-R may operate as servers; the same would be true for the hybridnetwork device 506; in the case of the special-purpose network device502, one or more such servers could also be run on a virtualizationlayer executed by the compute resource(s) 512); in which case theservers are said to be co-located with the VNEs of that ND.

A virtual network is a logical abstraction of a physical network (suchas that in FIG. 5A) that provides network services (e.g., L2 and/or L3services). A virtual network can be implemented as an overlay network(sometimes referred to as a network virtualization overlay) thatprovides network services (e.g., layer 2 (L2, data link layer) and/orlayer 3 (L3, network layer) services) over an underlay network (e.g., anL3 network, such as an Internet Protocol (IP) network that uses tunnels(e.g., generic routing encapsulation (GRE), layer 2 tunneling protocol(L2TP), IPSec) to create the overlay network).

A network virtualization edge (NVE) sits at the edge of the underlaynetwork and participates in implementing the network virtualization; thenetwork-facing side of the NVE uses the underlay network to tunnelframes to and from other NVEs; the outward-facing side of the NVE sendsand receives data to and from systems outside the network. A virtualnetwork instance (VNI) is a specific instance of a virtual network on aNVE (e.g., a NE/VNE on an ND, a part of a NE/VNE on a ND where thatNE/VNE is divided into multiple VNEs through emulation); one or moreVNIs can be instantiated on an NVE (e.g., as different VNEs on an ND). Avirtual access point (VAP) is a logical connection point on the NVE forconnecting external systems to a virtual network; a VAP can be physicalor virtual ports identified through logical interface identifiers (e.g.,a VLAN ID).

Examples of network services include: 1) an Ethernet LAN emulationservice (an Ethernet-based multipoint service similar to an InternetEngineering Task Force (IETF) Multiprotocol Label Switching (MPLS) orEthernet VPN (EVPN) service) in which external systems areinterconnected across the network by a LAN environment over the underlaynetwork (e.g., an NVE provides separate L2 VNIs (virtual switchinginstances) for different such virtual networks, and L3 (e.g., IP/MPLS)tunneling encapsulation across the underlay network); and 2) avirtualized IP forwarding service (similar to IETF IP VPN (e.g., BorderGateway Protocol (BGP)/MPLS IPVPN RFC 4364) from a service definitionperspective) in which external systems are interconnected across thenetwork by an L3 environment over the underlay network (e.g., an NVEprovides separate L3 VNIs (forwarding and routing instances) fordifferent such virtual networks, and L3 (e.g., IP/MPLS) tunnelingencapsulation across the underlay network)). Network services may alsoinclude quality of service capabilities (e.g., traffic classificationmarking, traffic conditioning and scheduling), security capabilities(e.g., filters to protect customer premises from network—originatedattacks, to avoid malformed route announcements), and managementcapabilities (e.g., full detection and processing).

FIG. 5D illustrates a network with a single network element on each ofthe NDs of FIG. 5A, and within this straight forward approach contrastsa traditional distributed approach (commonly used by traditionalrouters) with a centralized approach for maintaining reachability andforwarding information (also called network control), according to someembodiments of the invention. Specifically, FIG. 5D illustrates networkelements (NEs) 570A-H with the same connectivity as the NDs 500A-H ofFIG. 5A.

FIG. 5D illustrates that the distributed approach 572 distributesresponsibility for generating the reachability and forwardinginformation across the NEs 570A-H; in other words, the process ofneighbor discovery and topology discovery is distributed.

For example, where the special-purpose network device 502 is used, thecontrol communication and configuration module(s) 532A-R of the NDcontrol plane 524 typically include a reachability and forwardinginformation module to implement one or more routing protocols (e.g., anexterior gateway protocol such as Border Gateway Protocol (BGP) (RFC4271), Interior Gateway Protocol(s) (IGP) (e.g., Open Shortest PathFirst (OSPF) (RFC 2328 and 5340), Intermediate System to IntermediateSystem (IS-IS) (RFC 1142), Routing Information Protocol (RIP) (version 1RFC 1058, version 2 RFC 2453, and next generation RFC 2080)), LabelDistribution Protocol (LDP) (RFC 5036), Resource Reservation Protocol(RSVP) (RFC 2205, 2210, 2211, 2212, as well as RSVP-Traffic Engineering(TE): Extensions to RSVP for LSP Tunnels RFC 3209, GeneralizedMulti-Protocol Label Switching (GMPLS) Signaling RSVP-TE RFC 3473, RFC3936, 4495, and 4558)) that communicate with other NEs to exchangeroutes, and then selects those routes based on one or more routingmetrics. Thus, the NEs 570A-H (e.g., the compute resource(s) 512executing the control communication and configuration module(s) 532A-R)perform their responsibility for participating in controlling how data(e.g., packets) is to be routed (e.g., the next hop for the data and theoutgoing physical NI for that data) by distributively determining thereachability within the network and calculating their respectiveforwarding information. Routes and adjacencies are stored in one or morerouting structures (e.g., Routing Information Base (RIB), LabelInformation Base (LIB), one or more adjacency structures) on the NDcontrol plane 524. The ND control plane 524 programs the ND forwardingplane 526 with information (e.g., adjacency and route information) basedon the routing structure(s). For example, the ND control plane 524programs the adjacency and route information into one or more forwardingtable(s) 534A-R (e.g., Forwarding Information Base (FIB), LabelForwarding Information Base (LFIB), and one or more adjacencystructures) on the ND forwarding plane 526. For layer 2 forwarding, theND can store one or more bridging tables that are used to forward databased on the layer 2 information in that data. While the above exampleuses the special-purpose network device 502, the same distributedapproach 572 can be implemented on the general purpose network device504 and the hybrid network device 506.

FIG. 5D illustrates that a centralized approach 574 (also known assoftware defined networking (SDN)) that decouples the system that makesdecisions about where traffic is sent from the underlying systems thatforwards traffic to the selected destination. The illustratedcentralized approach 574 has the responsibility for the generation ofreachability and forwarding information in a centralized control plane576 (sometimes referred to as a SDN control module, controller, networkcontroller, OpenFlow controller, SDN controller, control plane node,network virtualization authority, or management control entity), andthus the process of neighbor discovery and topology discovery iscentralized. The centralized control plane 576 has a south boundinterface 582 with a data plane 580 (sometime referred to theinfrastructure layer, network forwarding plane, or forwarding plane(which should not be confused with a ND forwarding plane)) that includesthe NEs 570A-H (sometimes referred to as switches, forwarding elements,data plane elements, or nodes). The centralized control plane 576includes a network controller 578, which includes a centralizedreachability and forwarding information module 579 that determines thereachability within the network and distributes the forwardinginformation to the NEs 570A-H of the data plane 580 over the south boundinterface 582 (which may use the OpenFlow protocol). Thus, the networkintelligence is centralized in the centralized control plane 576executing on electronic devices that are typically separate from theNDs.

For example, where the special-purpose network device 502 is used in thedata plane 580, each of the control communication and configurationmodule(s) 532A-R of the ND control plane 524 typically include a controlagent that provides the VNE side of the south bound interface 582. Inthis case, the ND control plane 524 (the compute resource(s) 512executing the control communication and configuration module(s) 532A-R)performs its responsibility for participating in controlling how data(e.g., packets) is to be routed (e.g., the next hop for the data and theoutgoing physical NI for that data) through the control agentcommunicating with the centralized control plane 576 to receive theforwarding information (and in some cases, the reachability information)from the centralized reachability and forwarding information module 579(it should be understood that in some embodiments of the invention, thecontrol communication and configuration module(s) 532A-R, in addition tocommunicating with the centralized control plane 576, may also play somerole in determining reachability and/or calculating forwardinginformation—albeit less so than in the case of a distributed approach;such embodiments are generally considered to fall under the centralizedapproach 574, but may also be considered a hybrid approach).

While the above example uses the special-purpose network device 502, thesame centralized approach 574 can be implemented with the generalpurpose network device 504 (e.g., each of the VNE 560A-R performs itsresponsibility for controlling how data (e.g., packets) is to be routed(e.g., the next hop for the data and the outgoing physical NI for thatdata) by communicating with the centralized control plane 576 to receivethe forwarding information (and in some cases, the reachabilityinformation) from the centralized reachability and forwardinginformation module 579; it should be understood that in some embodimentsof the invention, the VNEs 560A-R, in addition to communicating with thecentralized control plane 576, may also play some role in determiningreachability and/or calculating forwarding information—albeit less sothan in the case of a distributed approach) and the hybrid networkdevice 506. In fact, the use of SDN techniques can enhance the NFVtechniques typically used in the general purpose network device 504 orhybrid network device 506 implementations as NFV is able to support SDNby providing an infrastructure upon which the SDN software can be run,and NFV and SDN both aim to make use of commodity server hardware andphysical switches.

FIG. 5D also shows that the centralized control plane 576 has a northbound interface 584 to an application layer 586, in which residesapplication(s) 588. The centralized control plane 576 has the ability toform virtual networks 592 (sometimes referred to as a logical forwardingplane, network services, or overlay networks (with the NEs 570A-H of thedata plane 580 being the underlay network)) for the application(s) 588.Thus, the centralized control plane 576 maintains a global view of allNDs and configured NEs/VNEs, and it maps the virtual networks to theunderlying NDs efficiently (including maintaining these mappings as thephysical network changes either through hardware (ND, link, or NDcomponent) failure, addition, or removal).

While FIG. 5D shows the distributed approach 572 separate from thecentralized approach 574, the effort of network control may bedistributed differently or the two combined in certain embodiments ofthe invention. For example: 1) embodiments may generally use thecentralized approach (SDN) 574, but have certain functions delegated tothe NEs (e.g., the distributed approach may be used to implement one ormore of fault monitoring, performance monitoring, protection switching,and primitives for neighbor and/or topology discovery); or 2)embodiments of the invention may perform neighbor discovery and topologydiscovery via both the centralized control plane and the distributedprotocols, and the results compared to raise exceptions where they donot agree. Such embodiments are generally considered to fall under thecentralized approach 574, but may also be considered a hybrid approach.

While FIG. 5D illustrates the simple case where each of the NDs 500A-Himplements a single NE 570A-H, it should be understood that the networkcontrol approaches described with reference to FIG. 5D also work fornetworks where one or more of the NDs 500A-H implement multiple VNEs(e.g., VNEs 530A-R, VNEs 560A-R, those in the hybrid network device506). Alternatively or in addition, the network controller 578 may alsoemulate the implementation of multiple VNEs in a single ND.Specifically, instead of (or in addition to) implementing multiple VNEsin a single ND, the network controller 578 may present theimplementation of a VNE/NE in a single ND as multiple VNEs in thevirtual networks 592 (all in the same one of the virtual network(s) 592,each in different ones of the virtual network(s) 592, or somecombination). For example, the network controller 578 may cause an ND toimplement a single VNE (a NE) in the underlay network, and thenlogically divide up the resources of that NE within the centralizedcontrol plane 576 to present different VNEs in the virtual network(s)592 (where these different VNEs in the overlay networks are sharing theresources of the single VNE/NE implementation on the ND in the underlaynetwork).

On the other hand, FIGS. 5E and 5F respectively illustrate exemplaryabstractions of NEs and VNEs that the network controller 578 may presentas part of different ones of the virtual networks 592. FIG. 5Eillustrates the simple case of where each of the NDs 500A-H implements asingle NE 570A-H (see FIG. 5D), but the centralized control plane 576has abstracted multiple of the NEs in different NDs (the NEs 570A-C andG-H) into (to represent) a single NE 570I in one of the virtualnetwork(s) 592 of FIG. 5D, according to some embodiments of theinvention. FIG. 5E shows that in this virtual network, the NE 570I iscoupled to NE 570D and 570F, which are both still coupled to NE 570E.

FIG. 5F illustrates a case where multiple VNEs (VNE 570A.1 and VNE570H.1) are implemented on different NDs (ND 500A and ND 500H) and arecoupled to each other, and where the centralized control plane 576 hasabstracted these multiple VNEs such that they appear as a single VNE570T within one of the virtual networks 592 of FIG. 5D, according tosome embodiments of the invention. Thus, the abstraction of a NE or VNEcan span multiple NDs.

While some embodiments of the invention implement the centralizedcontrol plane 576 as a single entity (e.g., a single instance ofsoftware running on a single electronic device), alternative embodimentsmay spread the functionality across multiple entities for redundancyand/or scalability purposes (e.g., multiple instances of softwarerunning on different electronic devices).

Similar to the network device implementations, the electronic device(s)running the centralized control plane 576, and thus the networkcontroller 578 including the centralized reachability and forwardinginformation module 579, may be implemented a variety of ways (e.g., aspecial purpose device, a general-purpose (e.g., COTS) device, or hybriddevice). These electronic device(s) would similarly include computeresource(s), a set or one or more physical NICs, and a non-transitorymachine-readable storage medium having stored thereon the centralizedcontrol plane software. For instance, FIG. 6 illustrates, a generalpurpose control plane device 604 including hardware 640 comprising a setof one or more processor(s) 642 (which are often COTS processors) andnetwork interface controller(s) 644 (NICs; also known as networkinterface cards) (which include physical NIs 646), as well asnon-transitory machine readable storage media 648 having stored thereincentralized control plane (CCP) software 650.

In embodiments that use compute virtualization, the processor(s) 642typically execute software to instantiate a virtualization layer 654 andsoftware container(s) 662A-R (e.g., with operating system-levelvirtualization, the virtualization layer 654 represents the kernel of anoperating system (or a shim executing on a base operating system) thatallows for the creation of multiple software containers 662A-R(representing separate user space instances and also calledvirtualization engines, virtual private servers, or jails) that may eachbe used to execute a set of one or more applications; with fullvirtualization, the virtualization layer 654 represents a hypervisor(sometimes referred to as a virtual machine monitor (VMM)) or ahypervisor executing on top of a host operating system, and the softwarecontainers 662A-R each represent a tightly isolated form of softwarecontainer called a virtual machine that is run by the hypervisor and mayinclude a guest operating system; with para-virtualization, an operatingsystem or application running with a virtual machine may be aware of thepresence of virtualization for optimization purposes). Again, inembodiments where compute virtualization is used, during operation aninstance of the CCP software 650 (illustrated as CCP instance 676A) isexecuted within the software container 662A on the virtualization layer654. In embodiments where compute virtualization is not used, the CCPinstance 676A on top of a host operating system is executed on the “baremetal” general purpose control plane device 604. The instantiation ofthe CCP instance 676A, as well as the virtualization layer 654 andsoftware containers 662A-R if implemented, are collectively referred toas software instance(s) 652.

In some embodiments, the CCP instance 676A includes a network controllerinstance 678. The network controller instance 678 includes a centralizedreachability and forwarding information module instance 679 (which is amiddleware layer providing the context of the network controller 578 tothe operating system and communicating with the various NEs), and an CCPapplication layer 680 (sometimes referred to as an application layer)over the middleware layer (providing the intelligence required forvarious network operations such as protocols, network situationalawareness, and user-interfaces). At a more abstract level, this CCPapplication layer 680 within the centralized control plane 576 workswith virtual network view(s) (logical view(s) of the network) and themiddleware layer provides the conversion from the virtual networks tothe physical view.

The centralized control plane 576 transmits relevant messages to thedata plane 580 based on CCP application layer 680 calculations andmiddleware layer mapping for each flow. A flow may be defined as a setof packets whose headers match a given pattern of bits; in this sense,traditional IP forwarding is also flow-based forwarding where the flowsare defined by the destination IP address for example; however, in otherimplementations, the given pattern of bits used for a flow definitionmay include more fields (e.g., 10 or more) in the packet headers.Different NDs/NEs/VNEs of the data plane 580 may receive differentmessages, and thus different forwarding information. The data plane 580processes these messages and programs the appropriate flow informationand corresponding actions in the forwarding tables (sometime referred toas flow tables) of the appropriate NE/VNEs, and then the NEs/VNEs mapincoming packets to flows represented in the forwarding tables andforward packets based on the matches in the forwarding tables.

Standards such as OpenFlow define the protocols used for the messages,as well as a model for processing the packets. The model for processingpackets includes header parsing, packet classification, and makingforwarding decisions. Header parsing describes how to interpret a packetbased upon a well-known set of protocols. Some protocol fields are usedto build a match structure (or key) that will be used in packetclassification (e.g., a first key field could be a source media accesscontrol (MAC) address, and a second key field could be a destination MACaddress).

Packet classification involves executing a lookup in memory to classifythe packet by determining which entry (also referred to as a forwardingtable entry or flow entry) in the forwarding tables best matches thepacket based upon the match structure, or key, of the forwarding tableentries. It is possible that many flows represented in the forwardingtable entries can correspond/match to a packet; in this case the systemis typically configured to determine one forwarding table entry from themany according to a defined scheme (e.g., selecting a first forwardingtable entry that is matched). Forwarding table entries include both aspecific set of match criteria (a set of values or wildcards, or anindication of what portions of a packet should be compared to aparticular value/values/wildcards, as defined by the matchingcapabilities—for specific fields in the packet header, or for some otherpacket content), and a set of one or more actions for the data plane totake on receiving a matching packet. For example, an action may be topush a header onto the packet, for the packet using a particular port,flood the packet, or simply drop the packet. Thus, a forwarding tableentry for IPv4/IPv6 packets with a particular transmission controlprotocol (TCP) destination port could contain an action specifying thatthese packets should be dropped.

Making forwarding decisions and performing actions occurs, based uponthe forwarding table entry identified during packet classification, byexecuting the set of actions identified in the matched forwarding tableentry on the packet.

However, when an unknown packet (for example, a “missed packet” or a“match-miss” as used in OpenFlow parlance) arrives at the data plane580, the packet (or a subset of the packet header and content) istypically forwarded to the centralized control plane 576. Thecentralized control plane 576 will then program forwarding table entriesinto the data plane 580 to accommodate packets belonging to the flow ofthe unknown packet. Once a specific forwarding table entry has beenprogrammed into the data plane 580 by the centralized control plane 576,the next packet with matching credentials will match that forwardingtable entry and take the set of actions associated with that matchedentry.

A network interface (NI) may be physical or virtual; and in the contextof IP, an interface address is an IP address assigned to a NI, be it aphysical NI or virtual NI. A virtual NI may be associated with aphysical NI, with another virtual interface, or stand on its own (e.g.,a loopback interface, a point-to-point protocol interface). A NI(physical or virtual) may be numbered (a NI with an IP address) orunnumbered (a NI without an IP address). A loopback interface (and itsloopback address) is a specific type of virtual NI (and IP address) of aNE/VNE (physical or virtual) often used for management purposes; wheresuch an IP address is referred to as the nodal loopback address. The IPaddress(es) assigned to the NI(s) of a ND are referred to as IPaddresses of that ND; at a more granular level, the IP address(es)assigned to NI(s) assigned to a NE/VNE implemented on a ND can bereferred to as IP addresses of that NE/VNE.

While the flow diagrams in the figures show a particular order ofoperations performed by certain embodiments of the invention, it shouldbe understood that such order is exemplary (e.g., alternativeembodiments may perform the operations in a different order, combinecertain operations, overlap certain operations, etc.).

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, can be practiced with modificationand alteration within the spirit and scope of the appended claims. Thedescription is thus to be regarded as illustrative instead of limiting.

What is claimed is:
 1. A method implemented by a network node, ofsignaling a maximum segment identifier depth (MSD) of the network nodeto a centralized controller, the method comprising: encoding the MSDinto an attribute Type Length Value (TLV) including a type, a length anda value, wherein the type indicates the type of the MSD, the lengthindicates the length of the MSD and the value is a number that is amaximum number of segment routing (SR) labels that the network node iscapable of imposing, wherein an SR label is used for steering a packetthrough an SR network, wherein the type of the attribute TLV indicatesthat the MSD is a link MSD and the value of the attribute TLV is alowest MSD supported by a link of the network node; and transmitting theattribute TLV to the centralized controller, wherein the centralizedcontroller is to use the attribute TLV to compute an SR tunnel and thelabel stack depth of the SR tunnel does not exceed the MSD supported bythe network node.
 2. The method of claim 1, wherein the network node isan ingress node of the SR tunnel.
 3. The method of claim 1, wherein theattribute TLV is a border gateway protocol Link State (BGP-LS) extensionmessage.
 4. The method of claim 1, wherein the network node does notsupport Path Computation Element Communication Protocol (PCEP).
 5. Themethod of claim 1, wherein the network node does not participate inInterior Gateway Protocol(s) (IGP).
 6. A network node for signaling amaximum segment identifier depth (MSD) of the network node to acentralized controller, comprising: a processor and a memory, saidmemory containing instructions executable by the processor to cause thenetwork node to: encode the MSD into an attribute Type Length Value(TLV) including a type, a length and a value, wherein the type indicatesthe type of the MSD, the length indicates the length of the MSD and thevalue is a number that is a maximum number of segment routing (SR)labels that the network node is capable of imposing, wherein an SR labelis used for steering a packet through an SR network, wherein the type ofthe attribute TLV indicates that the MSD is a link MSD and the value ofthe attribute TLV is a lowest MSD supported by a link of the networknode; and transmit the attribute TLV to the centralized controller,wherein the centralized controller is to use the attribute TLV tocompute an SR tunnel and the label stack depth of the SR tunnel does notexceed the MSD supported by the network node.
 7. The network node ofclaim 6, wherein the network node is an ingress node of the SR tunnel.8. The network node of claim 6, wherein the attribute TLV is a bordergateway protocol Link State (BGP-LS) extension message.
 9. The networknode of claim 6, wherein the network node does not support PathComputation Element Communication Protocol (PCEP).
 10. The network nodeof claim 6, wherein the network node does not participate in InteriorGateway Protocol(s) (IGP).
 11. A non-transitory machine-readable storagemedium that provides instructions that, if executed by a processor of anetwork node, will cause said processor to perform operationscomprising: encoding a maximum segment identifier depth (MSD) of thenetwork node into an attribute Type Length Value (TLV) including a type,a length and a value, wherein the type indicates the type of the MSD,the length indicates the length of the MSD and the value is a numberthat is a maximum number of segment routing (SR) labels that the networknode is capable of imposing, wherein an SR label is used for steering apacket through an SR network, wherein the type of the attribute TLVindicates that the MSD is a link MSD and the value of the attribute TLVis a lowest MSD supported by a link of the network node; andtransmitting the attribute TLV to a centralized controller, wherein thecentralized controller is to use the attribute TLV to compute an SRtunnel and the label stack depth of the SR tunnel does not exceed theMSD supported by the network node.
 12. The non-transitorymachine-readable storage medium of claim 11, wherein the network node isan ingress node of the SR tunnel.
 13. The non-transitorymachine-readable storage medium of claim 11, wherein the attribute TLVis a border gateway protocol Link State (BGP-LS) extension message. 14.The non-transitory machine-readable storage medium of claim 11, whereinthe network node does not support Path Computation Element CommunicationProtocol (PCEP).
 15. The non-transitory machine-readable storage mediumof claim 11, wherein the network node does not participate in InteriorGateway Protocol(s) (IGP).
 16. A method in a network controller, themethod comprising: receiving from a network node an attribute TypeLength Value (TLV) including a type, a length and a value; decoding theattribute TLV, to extract a maximum segment identifier depth (MSD) ofthe network node, wherein the type of the attribute TLV indicates thetype of the MSD, the length of the attribute TLV indicates the length ofthe MSD and the value of the attribute TLV is a number that is a maximumnumber of segment routing (SR) labels that the network node is capableof imposing, wherein an SR label is used for steering a packet throughan SR network, and wherein the type of the attribute TLV indicates thatthe MSD is a link MSD and the value of the attribute TLV is a lowest MSDsupported by a link of the network node; and computing, using the MSD,an SR tunnel including the network node, wherein the SR tunnel has alabel stack depth that is lower than or equal to the MSD of the networknode.
 17. The method of claim 16, wherein the network node is an ingressnode of the SR tunnel.
 18. The method of claim 16, wherein the attributeTLV is a border gateway protocol Link State (BGP-LS) extension message.19. The method of claim 16, wherein the network node does not supportPath Computation Element Communication Protocol (PCEP).
 20. The methodof claim 16, wherein the network node does not participate in InteriorGateway Protocol(s) (IGP).
 21. A centralized controller comprising: aprocessor and a memory, said memory containing instructions executableby the processor to cause the centralized controller to: receive from anetwork node an attribute Type Length Value (TLV) including a type, alength and a value; and decode the attribute TLV, to extract a maximumsegment identifier depth (MSD) of the network node, wherein the type ofthe attribute TLV indicates the type of the MSD, the length of theattribute TLV indicates the length of the MSD and the value of theattribute TLV is a number that is a maximum number of segment routing(SR) labels that the network node is capable of imposing, wherein an SRlabel is used for steering a packet through an SR network, and whereinthe type of the attribute TLV indicates that the MSD is a link MSD andthe value of the attribute TLV is a lowest MSD supported by a link ofthe network node; and compute, using the MSD, an SR tunnel including thenetwork node, wherein the SR tunnel has a label stack depth that islower than or equal to the MSD of the network node.
 22. The centralizedcontroller of claim 21, wherein the network node is an ingress node ofthe SR tunnel.
 23. The centralized controller of claim 21, wherein theattribute TLV is a border gateway protocol Link State (BGP-LS) extensionmessage.
 24. The centralized controller of claim 21, wherein the networknode does not support Path Computation Element Communication Protocol(PCEP).
 25. The centralized controller of claim 21, wherein the networknode does not participate in Interior Gateway Protocol(s) (IGP).
 26. Anon-transitory machine-readable storage medium that providesinstructions that, if executed by a processor of a centralizedcontroller, will cause said processor to perform operations comprising:receiving from a network node an attribute Type Length Value (TLV)including a type, a length and a value; decoding the attribute TLV, toextract a maximum segment identifier depth (MSD) of the network node,wherein the type of the attribute TLV indicates the type of the MSD, thelength of the attribute TLV indicates the length of the MSD and thevalue of the attribute TLV is a number that is a maximum number ofsegment routing (SR) labels that the network node is capable ofimposing, wherein an SR label is used for steering a packet through anSR network, and wherein the type of the attribute TLV indicates that theMSD is a link MSD and the value of the attribute TLV is a lowest MSDsupported by a link of the network node; and computing, using the MSD,an SR tunnel including the network node, wherein the SR tunnel has alabel stack depth that is lower than or equal to the MSD of the networknode.
 27. The non-transitory machine-readable storage medium of claim26, wherein the network node is an ingress node of the SR tunnel. 28.The non-transitory machine-readable storage medium of claim 26, whereinthe attribute TLV is a border gateway protocol Link State (BGP-LS)extension message.
 29. The non-transitory machine-readable storagemedium of claim 26, wherein the network node does not support PathComputation Element Communication Protocol (PCEP).
 30. Thenon-transitory machine-readable storage medium of claim 26, wherein thenetwork node does not participate in Interior Gateway Protocol(s) (IGP).